Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) passed by some children who were playing with dirt, and some of the Companions of the Prophet (peace and blessings be upon him) told them to stop. He (peace and blessings be upon him) said: "Leave them, for dirt is the springtime of children."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5754